Raised bed gardening is a popular method for gardeners in the Washington, DC area growing vegetables, especially in areas with poor or contaminated topsoil. Raised beds are essentially large planter boxes built above the soil grade. They are often one foot or more in depth and built using simple designs from treated lumber or cinderblocks directly on the soil surface. If raised beds are built directly on the soil, it is wise to rototill the soil under the beds to ensure drainage. Some gardeners place a breathable landscape fabric under their raised beds to better contain the media, though this can impede water movement over time. If there are rodent concerns, a layer of hardware cloth (metal mesh) can be placed at the bottom of the raised bed.
The location of your raised garden bed is critical. Since most herbs, vegetables, small fruits, and flowering plants require full sun exposure, situate your raised garden bed in a location that provides full sun for at least six hours per day. Always account for shadows created by taller neighboring structures. Ideally, you should construct the raised bed on an even surface where water does not pool, and near a water source.
Soil quality is just as important as light and water in raised bed gardening. In the DMV area, where heavy clay soils proliferate, soil media should not be derived primarily from native soil. That is not to say that native soils cannot be used as a component to the media.
Raised garden bed media can be purchased in bags, but that is expensive and often the media possesses a very low bulk density, having no sand or native topsoil. The end result is that larger plants may tip over, especially when laden with fruits or vegetables.
In most cases, it is cheaper and easier to obtain finished soil in bulk or make your own from bulk materials. The University of Maryland Cooperative Extension Service suggests a media containing an “organic matter content of 25%-50% by volume or 5%-15% by dry weight (as measured by a soil testing lab) and a pH of 6.2 to 6.8.”
If you are working with a bulk landscape material supplier, they will likely have an appropriate media containing a blend of topsoil, sand, and a source of organic matter like Bloom Woody Blend or compost. See Bloom’s retailer map for Bloom-infused topsoil options. Otherwise, a blend could be made from scratch using Bloom; see Bloom’s recommended blending ratios for its Woody Blend and Sandy Blend.
It’s always recommended to test (or ask for tests from the supplier) the materials you use in developing a raised bed media to make sure it meets your planting requirements. Our video on interpreting nutrient analyses is an excellent starting point to understanding the basics of testing; most soil testing labs will be able to help you understand your test results in the context of your planting goals and provide concrete recommendations for addressing any concerns in the soil. The University of District of Columbia, the University of Maryland, and Virginia Tech all offer low-cost soil testing for local residents.
Bloom’s Woody Blend contains up to 60% organic matter, which is even greater than most composts. Adding a soil amendment like Bloom, which is rich in organic matter, will enhance the soil’s water-holding capacity and soil aggregation, in turn, improving water and air movement. Bloom also provides slow-release nutrients and increases the soil’s cation exchange capacity, allowing it to better hold nutrients in the plants’ root zone.
Raised bed soils will benefit from periodic amending to replenish the organic matter lost over time. If plant growth is sluggish even after fertilizing, add two inches of Bloom’s Woody Blend or compost, then incorporate it to a six-inch depth or deeper. Soil can be amended in the spring or fall, depending on when you plan to plant. Many avid vegetable gardeners will apply soil amendments to their beds after they finish harvesting in the fall, then let the soil amendment further stabilize over winter before replanting in the spring. That said, a soil amendment does not usually have to be added every year. Always consider the requirements of the plants being established, as well as the soil characteristics, before adding a soil amendment, especially nutrient-rich ones.
Planting time is right around the corner, so be prepared with highly productive soils and soil amendments to maximize your garden’s success!
Ron Alexander is President of R. Alexander Associates, Inc. a consulting company specializing in product development for organic recycled products. He is a Horticulturist (Delaware Valley University) with 40 years of experience working with biosolids-based, compost and other organic recycled products. Ron is also a technical consultant and has completed various product trials with Bloom products.
